This invention relates to separating devices in general and more particularly to a device for the separation of magnetizable particles down to particle sizes below 1 .mu.m.
Separation of magnetizable particles down to sizes of 1 .mu.m from a flowing medium using a filter structure arranged in a filter space which is disposed between two parts of a ferromagnetic yoke of a magnetic device forming two magnetic poles in a magnetic field directed substantially parallel or anti-parallel to the flow direction of the medium in the region of the filter structure and which contains several wire screens which are arranged closely one behind the other as seen in the flow direction and at least approximately perpendicular to the flow direction of the medium, using the principle of high gradient magnetic separation technology is described in U.S. application Ser. No. 15,168. The screens are made of non-corroding, ferromagnetic material with predetermined mesh size and wire thickness.
In magnetic separating methods, the fact that, in a suitable magnetic field configuration, a magnetizable particle is subjected to a force which moves it or holds it against other forces attaching it such as, for instance, the force of gravity or, in a liquid medium, against hydrodynamic friction forces is utilized. Such separating methods are provided, for instance, for steam or cooling water loops in conventional as well as nuclear power plants. In the liquid or gaseous medium of these loops, particles which, in general, have been produced by corrosion are suspended. These particles are in part ferromagnetic such as a magnetite (Fe.sub.3 O.sub.4), partly antiferromagnetic such as hematite (.alpha.-Fe.sub.2 O.sub.3) or paramagnetic such as copper oxide (CuO). The magnetizability of these particles, which, in addition, appear in different sizes, is consequently different.
Very small ferromagnetic particles with particle diameters in the order of 1 .mu.m or also weakly magnetic, i.e., antiferromagnetic or paramagnetic particles can be filtered our magnetically with an appreciable degree of separation practically only with separating devices of the so-called high gradient magnetic separation technology (HGM technique) from a flowing medium (see, for instance, "Journal of Magnetism and Magnetic Materials", vol. 13, 1979, pages 1 to 10).
A corresponding HGM separating device is also described U.S. application Ser. No. 15,168. It contains a central filter space with a filter structure of a multiplicity of wire screens arranged closely together in a stack as seen in the flow direction. The screens are arranged perpendicular to the flow direction of the medium in a relatively strong magnetic field. This magnetic field is directed parallel or anti-parallel to the flow direction of the medium in the vicinity of the filter structure and causes there, for instance, a magnetic induction in the order of 1 Tesla. The thickness of the screen wires consisting of ferromagnetic material is very small and is, for instance, less than 0.1 mm. The magnetic field gradients produced at them are then consequently very high so that even weakly magnetizable particles can be filtered out with the separating device.
The central filter space of the known separating device, in which the filter structure of the wire screens is contained, is arranged between the ends of two pole pieces which are part of a yoke body of ferromagnetic material which serves to conduct the magnetic field caused by a magnet coil. The medium to be filtered is conducted either via holes in these pole pieces themselves or through a gap remaining between the pole pieces via ring-shaped chambers and into and out of the filter space. In the case of an axial inlet and outlet of the medium, relatively high flow velocities are obtained, however, in the pole pieces with holes, and inhomogeneous separation at the filter input over the filter cross section. Also with radial flow in and out of the medium, turbulence develops over the filter cross section which leads to uneven separation in the filter structure.
It is therefore an object of the present invention to improve the magnetic separating deviced mentioned at the outset in such a manner that particularly the inflow of the medium containing the particles to be separated into the filter structure is equalized and, at the same time, a reduction of the magnetic induction in the filter structure is prevented.